Dark Matter
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Definition
Dark Matter is matter that we cannot see.
It neither emits nor reflects any light. If we
can’t see it, how do we know it exists?
Scientists can measure dark matter
indirectly by observing its gravitational
effects in a variety of ways. Dark matter is
one of the most fascinating mysteries in
science.
Something isn’t right with:
• The rotation of galaxies.
• The appearance of far away galaxies.
• The behavior of galaxy clusters.
• The total amount of observed matter in the
universe.
What’s the solution?
Galaxies Rotate
Galaxies are collections
of billions of stars. Most
of the light from a galaxy
comes from its center.
This indicated that most
of the galaxies stars and
most of its mass is
concentrated at its center.
Under this scenario, we
should expect the stars in
the outer part of the
galaxy to rotate about the
center, and this is just
what we observe.
But . . .
But outer stars do not rotate correctly! If gravity causes galaxies to
rotate, as we assume it does, then outer stars should behave much
like the planets of our solar system. Inner planets rotate faster and
outer planets rotate slower. This is called Keplerian motion. (learn
more) In galaxies, however, both inner and outer stars rotate at
about the same speed.
Can our model be saved?
In order for gravity to
cause this type of
rotation, each galaxy
must be surrounded
by a super-massive
halo of matter. No
such halos, however,
can be seen. So we
conclude that they are
made of dark matter.
Dark matter
Who squashed the galaxies?
In 1995, the Hubble Space Telescope focused its
attention on a very small patch of sky near the Big
Dipper. It was able to see farther away (and further back
in time) than any other optical telescope in history. It saw
thousands of new galaxies. Many appeared squashed or
stretched out.
Gravitational Lensing
What could be strong enough
to crush a galaxy? It turns out
that just the images of the
galaxies were stretched out.
The culprit was gravity. The
light from these distant
galaxies was attracted to mass
between the galaxy and the
Earth. This attraction bent the
light and caused the galaxies
to look distorted, as if they
were being seen through a
crooked lens. (learn more)
Lensing by dark matter
Sometimes galaxies are lensed by other galaxies. Other times they
were lensed by invisible objects – dark matter. By measuring the
distortion of the galaxies, scientists were able to “weigh” the dark
matter. They found that it accounts for 90% of the mass of the
universe.
Galaxy Clusters
Galaxies have been
called the atoms of
the universe. Nearly
all the visible matter
in the universe is
found in galaxies
which are distributed
throughout space.
Galaxies are often
found in groups called
clusters.
More gravity
Radio astronomers have found
hot gas in the space between
galaxies in a cluster. This gas
produces a pressure that
pushes the galaxies apart.
The galaxies’ mutual
gravitational attraction causes
them to cling together. The
heavier the galaxies, the
stronger the gravitational
attraction.
So, are galaxies massive
enough to hang together?
Here’s one simple way to
mass a galaxy
Mass of galaxy = number of stars x average mass of star
It turns out that galaxies do not have enough
visible mass to stay grouped in clusters. The
extra mass they need must come from dark
matter.
The Universe
Cosmologists study the birth and death of the
universe. They also study its properties
including its shape. Recent observation indicate
that the universe is flat.
Curvature
According to general relativity, mass bends space.
Cosmologists have calculated the density necessary for
the universe to be flat – the critical density. If the universe
had a lower average density, it would have negative
curvature and an open shape. If the universe had a
greater density it would have positive curvature and a
closed shape.
Mass / Energy Density
Scientists have measured
the density of the universe
by studying clusters of
galaxies. We can call it
either mass density or
energy density. Einstein
proved that mass and
energy are the same with
his equation E = mc2. He’s
also the guy to blame for
this curved space-time stuff.
(learn more)
Dark Energy
Chart of the matter of the universe. In a
way, this chart is an embarrassment for
scientists. We are only able to account for
4% of the matter in the universe.
Even when they include
dark matter, the measured
values for the density of the
universe are far smaller
than the calculated value
for the critical density.
Once again, we’re missing
something. The missing
quantity is called dark
energy. It’s different from
dark matter, but just as
mysterious. (learn more)
The Solution?
Many exotic particles have been proposed as
candidates for dark matter including massive
neutrinos, weakly interacting massive particles
(WIMP’s), massive compact halo objects
(MACHO’s), black holes, and brown dwarfs.
(learn more) It is unlikely that one explanation
will satisfy everyone. When looking at galaxy
rotation, it appears that dark matter occurs in
halos around galaxies. But when looking at
gravitational lensing and clusters, most of the
dark matter appears to be smeared out in
between galaxies.
More confusion
If the particles that make up dark matter are
small, then dark matter is said to be hot. If the
particles are large, then it is called cold.
Theories with cold dark matter have more
success explaining how galaxies formed.
Theories with hot dark matter do a better job
explaining the origins of clusters and
superclusters. Recent experiments suggest that
dark matter is cold, but some researchers
believe that the universe contains a mix of both
hot and cold dark matter.
Continued Research
What is the universe made of and what is
its fate? These are active areas of
research. Find out more at
• WMAP
• Haystack
• Chandra
• Bell Labs